Amazon.com, still in bitter rivalry with Apple over their competing e-book and application store businesses, has struck back by today announcing an in-app purchasing service, allowing Amazon Appstore developers to offer digital subscriptions and content within apps and games on Android-running Kindle devices.

Millions of Kindle Fires and other Android devices will be able to buy content with Amazon's 1-Click service through many popular apps and games, including those from Adobe, Disney, Dow Jones, Gameloft, and the New York Post.

The two companies continue with an acrimonious and bitter battle over e-book pricing, and the approaches to their respective application stores.

Despite the retail giant having no direct involvement in the case, with it being seen as more of a victim than an instigator, Apple along with five major international publishers are being investigated by EU and U.S. antitrust authorities for conspiring to fix e-book prices.

The case revolves around Amazon's 'wholesale' pricing model, allowing e-book writers to sell their content at a loss, compared to Apple's 'agency' model which forces a 30 percent cut of all e-book sales.

Apple, the market leader in application providing, forced a change to its App Store, which has over half a million applications, disallowing in-app purchases.

In mid-July, Apple changed its policy to force major applications and content publishers to give the maker of shiny rectangles a 30 percent cut of the profits made. Ultimately, it meant iOS developers had to change their applications and issue an update removing in-app purchases, or risk being thrown out of the lucrative App Store, which could kill revenues.

Apple infamously sued Amazon over the "App Store" trademark, claiming it owned the name. The iPad and iPhone maker, both devices of which have an application store pre-installed, sought a preliminary injunction against Amazon from using the term, but was ultimately denied in 2011.

Amazon prevailed, and brought out its own application store for Android in March 2011, which now has thousands of apps and games available.

"With the Amazon Appstore for Android In-App Purchasing solution, you can reach customers with existing accounts who have already bought apps, including millions of Kindle Fire customers," the company said on its developer blog.

"Our simple, secure, and trusted 1-Click purchase experience is easy for customers to use, increasing conversion rates for purchases within your app. Plus, we designed our in-app purchasing (IAP) solution to be simple and easy to integrate so you can be up and running quickly."